Some transmission type image display apparatuses such as liquid crystal display apparatuses use a direct surface light source device as a backlight. In recent years, direct surface light source devices having a plurality of light emitting elements as the light source have been used.
For example, a direct surface light source device includes a substrate, a plurality of light emitting elements, a plurality of light flux controlling members (lens) and a light diffusion member. Each of the light emitting elements is, for example, a light-emitting diode (LED) such as a white light-emitting diode. The light emitting elements are disposed on the substrate in a matrix. The light flux controlling member that spreads the light of the light emitting element in the surface direction of the substrate is disposed over each light emitting element. The light emitted from the light flux controlling member is diffused by the light diffusion member so as to illuminate an illumination member (for example, a liquid crystal panel) in a planar fashion.
FIGS. 1A to 1C illustrate a configuration of a conventional light flux controlling member. FIG. 1A is a perspective view as viewed from a rear side, FIG. 1B is a perspective view illustrating a cross-section as viewed from a rear side, and FIG. 1C is a sectional view. It is to be noted that the leg part provided on the rear side is omitted in FIGS. 1A and 1B. As illustrated in FIGS. 1A to 1C, conventional light flux controlling member 20 includes incidence surface 22 on which light emitted from a light emitting element is incident, and emission surface 24 configured to emit light incident on incidence surface 22 to the outside. Incidence surface 22 is a surface recessed with respect to the light emitting element, and is formed to face the light emitting surface of the light emitting element.
FIGS. 2A and 2B illustrate light paths of light flux controlling member 20. FIG. 2A illustrates light paths of light beams emitted from a light emission center of light emitting element 10 at an emission angle of 30 degrees, and FIG. 2B illustrates light paths of light beams emitted from a light emission center of light emitting element 10 at an emission angle of 40 degrees. Here, the “emission angle” is an angle (θ in FIG. 2A) of a light beam with respect to light axis LA of light emitting element 10. It is to be noted that the leg part provided on the rear side is omitted in FIGS. 2A and 2B.
As illustrated in FIGS. 2A and 2B, light emitted from light emitting element 10 enters light flux controlling member 20 from incidence surface 22. The light having entered light flux controlling member 20 reaches emission surface 24 and is emitted from emission surface 24 to the outside (solid line arrow). At this time, the light is refracted by the shape of emission surface 24, and accordingly the travelling direction of the light is controlled. Meanwhile, a part of the light having reached emission surface 24 is reflected (fresnel-reflected) at emission surface 24 and reaches rear surface 26 opposite to the substrate on which light emitting element 10 is mounted (broken line arrow). When the light having reached rear surface 26 is reflected at rear surface 26, the light travelling toward a part right above light flux controlling member 20 is excessively increased, and consequently non-uniformity of the distribution (luminance unevenness) of the luminance of the light applied by the light-emitting device is caused. In addition, when the light having reached rear surface 26 is emitted from rear surface 26, the light is absorbed by the substrate and consequently significant loss of light results. In view of this, PTL 1 proposes a light flux controlling member for solving the above-mentioned problems.
FIGS. 3A to 3C illustrate a configuration of light flux controlling member disclosed in PTL 1. FIG. 3A is a perspective view as viewed from a rear side, FIG. 3B is a perspective view illustrating a cross section as viewed from a rear side, and FIG. 3C is a sectional view. It is to be noted that the leg part provided on the rear side is omitted in FIGS. 3A and 3B. As illustrated in FIGS. 3A to 3C, in light flux controlling member 30 disclosed in PTL 1, a recess is formed on rear surface 26. The recess includes inclined surface 32 which is provided on the outer side, and parallel surface 34 which is substantially parallel to central axis CA and is provided on the inner side. Inclined surface 32 is rotationally symmetrical (circularly symmetrical) about central axis CA of light flux controlling member 30, and is tilted at a predetermined angle (for example, 45 degrees) to a virtual line orthogonal to central axis CA.
FIGS. 4A and 4B illustrate light paths of light flux controlling member 30. FIG. 4A illustrates light paths of light beams emitted from a light emission center of light emitting element 10 at an emission angle of 30 degrees, and FIG. 4B illustrates light paths of light beams emitted from a light emission center of light emitting element 10. It is to be noted that the leg part provided on the rear side is omitted in FIGS. 4A and 4B. As illustrated in FIGS. 4A and 4B, light fresnel-reflected at emission surface 24 reaches a predetermined portion of rear surface 26. By forming inclined surface 32 in the above-mentioned predetermined portion, at least a part of the light reaching inclined surface 32 can be reflected in the lateral direction (see FIGS. 4A and 4B).
In this manner, in light flux controlling member 30 disclosed in PTL 1, light reflected at emission surface 24 does not tend to be directed toward a part right above light flux controlling member 30, or does not tend to be absorbed by the substrate. Accordingly, a light-emitting device having light flux controlling member 30 disclosed in PTL 1 can uniformly and efficiently emit light in comparison with a conventional light-emitting device having light flux controlling member 20.